Skeletal muscle regeneration comprises several overlapping cellular processes, including inflammation and myogenesis. Prostaglandins (PGs) may regulate muscle regeneration, because they modulate inflammation and are involved in various stages of myogenesis in vitro. PG synthesis is catalyzed by different isoforms of cyclooxygenase (COX), which are inhibited by nonsteroidal anti-inflammatory drugs. Although experiments employing nonsteroidal anti-inflammatory drugs have implicated PGs in tissue repair, how PGs regulate muscle regeneration remains unclear, and the potentially distinct roles of different COX isoforms have not been investigated. To address these questions, a localized freeze injury was induced in the tibialis anterior muscles of mice chronically treated with either a COX-1- or COX-2-selective inhibitor (SC-560 and SC-236, respectively), starting before injury. The size of regenerating myofibers was analyzed at time points up to 5 wk after injury and found to be decreased by SC-236 and in COX-2(-/-) muscles, but unaffected by SC-560. In contrast, SC-236 had no effect on myofiber growth when administered starting 7 days after injury. The attenuation of myofiber growth by SC-236 treatment and in COX-2(-/-) muscles is associated with decreases in the number of myoblasts and intramuscular inflammatory cells at early times after injury. Together, these data suggest that COX-2-dependent PG synthesis is required during early stages of muscle regeneration and thus raise caution about the use of COX-2-selective inhibitors in patients with muscle injury or disease.

Loss of muscle mass occurs with disease, injury, aging, and inactivity. Restoration of normal muscle mass depends on myofiber growth, the regulation of which is incompletely understood. Cyclooxygenase (COX)-2 is one of two isoforms of COX that catalyzes the synthesis of prostaglandins, paracrine hormones that regulate diverse physiological and pathophysiological processes. Previously, we demonstrated that the COX-2 pathway regulates early stages of myofiber growth during muscle regeneration. However, whether the COX-2 pathway plays a common role in adult myofiber growth or functions specifically during muscle regeneration is unknown. Therefore, we examined the role of COX-2 during myofiber growth following atrophy in mice. Muscle atrophy was induced by hindlimb suspension (HS) for 2 wk, followed by a reloading period, during which mice were treated with either the COX-2-selective inhibitor SC-236 (6 mg x kg(-1) x day(-1)) or vehicle. COX-2 protein was expressed and SC-236 attenuated myofiber growth during reloading in both soleus and plantaris muscles. Attenuated myofiber growth in the soleus was associated with both decreased myonuclear addition and decreased inflammation, whereas neither of these processes mediated the effects of SC-236 on plantaris growth. In addition, COX-2(-/-) satellite cells exhibited impaired activation/proliferation in vitro, suggesting direct regulation of muscle cell activity by COX-2. Together, these data suggest that the COX-2 pathway plays a common regulatory role during various types of muscle growth via multiple mechanisms.

Satellite cells are stem cells that are critical for the formation and growth of skeletal muscle during myogenesis. To differentiate and fuse, proliferating satellite cells or myoblasts must migrate and establish stable cell-cell contacts. However, the factors that regulate myoblast migration and fusion are not understood completely. We have identified PGI2 as a novel regulator of myogenesis in vitro. PGI2 is a member of the family of prostaglandins (PG), autocrine/paracrine signaling molecules synthesized via the cyclooxygenase-1 and -2 pathways. Primary mouse muscle cells both secrete PGI2 and express the PGI2 receptor, IP, at various stages of myogenesis. Using genetic and pharmacological approaches, we show that PGI2 is a negative regulator of myoblast migration that also enhances cell fusion. Thus, PGI2 may act as a "brake" on migrating cells to facilitate cell-cell contact and fusion. Together, our results highlight the importance of the balance between positive and negative regulators in cell migration and myogenesis. This work may have implications for migration of other populations of adult stem cells and/or cells that undergo fusion.

This paper describes experiments that teach junior/senior undergraduate students about some of the uses of circular dichroism in characterizing several aspects of globular protein structure. The four experiments are (i) qualitative comparison of CD spectra for globular proteins with different amounts of secondary and tertiary structures, (ii) characterization of secondary and tertiary structure of the A state of cytochrome c, which is believed to resemble the molten globule, (iii) kinetics of lysozyme denaturation, and (iv) helicogenesis of concanavalin A, whereby the predominantly β-sheet protein structure is converted into mostly α-helix. These experiments have the pedagogical advantage of giving students experience with a spectroscopic method that provides more important details about protein structures than is provided by most routine spectroscopic methods.